Bulldog clips. This isn't a solution for beloved original art or family heirlooms, but it's an easy trick for prints and photos, which can be switched in and out quickly to transform the feel of a room with minimal effort. The simple metal clips have a minimalist, timeless feel, and they look especially good in rooms with a casual, vintage-y, or industrial edge.
You can find bulldog clips at almost any store that sells office supplies, and at Amazon where you can score three dozen for under ten bucks.
For more ideas on easy (and noncommittal) ways to hang your art, check out this past post on Rotating Artwork Displays, 5 Ways.